The shedding of rat rod outer segment (ROS) discs, by a process of phagocytosis, is a circadian phenomenon characterized morphologically by the release of ROS discs into the pigment epithelium. This process occurs over a period of a few hours beginning about two hours after the start of the light period. Prior to the appearance of phagosomes in the pigment epithelium and coincident with onset of the daily light period, there is increased phospholipid synthesis, particularly phosphatidyl choline and phosphatidyl ethanolamine. This occurs when animals entrained to 12 hours dark, 12 hours light, are kept in the dark. Phosphatidylinositol synthesis appears to increase when animals are exposed to light at the initiation of the light period. An in vitro assay system employing labelled glycerol, incubation of retinas for 20 minutes, and separation of phospholipid classes by two-dimensional thin layer chromatography have led to the detection of this event which may be correlated with shedding.